We depend on our friends more than ever. Or at least sociologists tell us we do. Lovers come and go, family ties are looser and marriage is for losers (in terms of risk exposure). As for our neighbours, they are unreliable and unchosen - hardly the rocks we need to cling to in a floating world. Friends are now the ones you turn to in times of need, who forgive your failings, bring happiness, lend emotional support.

But this reliance comes at a cost. "A tremendous burden is being placed upon friendship," argues Mark Vernon, the author of the new book The Philosophy of Friendship. "More and more is being asked of this voluntary, informal, personal relationship. It is commonplace for sociologists to note that institutions like marriage, kinship, class unions and corporations are losing their stickiness. As their power to hold society together moderates, so, they say, people are turning to friendship to support them and secure their sense of place in the world."

But is friendship up to the job of providing security? After all, we arguably live in times in which fear of commitment poisons every possible human relationship - sexual, professional, familial. Why should friendship be any different? We've become temperamental Taoists, going with the flow, suspicious of all things rooted or permanent. One might even argue that there is a contradiction at the heart of modern friendship: not only is friendship bearing an impossible burden, but it, too, has changed in a manner that makes it impossible to carry that weight. It is no longer about quality but quantity, as creative networkers minimise their risk exposure and maximise utility by creatively destroying and making new bonds. The soul-expanding, ardent spiritual intimacy of Aristotle or Emerson's ideal friendships may continue to exist, but that is a rare phenomenon.

The ties of friendship, like all human bonds, are loosening. Relationships are like Ribena for the uprooted and anxious of our late modern age - taken undiluted, they are nauseating. Our deepest wish is to prevent our relationships from curdling (that, we fear, is what marriages and families used to be about). That's not to say that we're all hipster SDCs (semi-detached couples), the self-styled romantic revolutionaries who scandalised the sociologist philosopher Zygmunt Bauman for wanting separate pads and a Rolodex filled with ready lovers. We don't all want to pour water on troubled relationships, and thus the SDCs provoke as much hostility as identification. We don't know what we want, but, because we're desperately anxious and insecure, we want it yesterday.

If we have SDCs, why not semi-detached friends? SDFs are everywhere. And they work harder at making friends than ever before - jabbering into mobile phones, addictively texting, leaping from one chat room to another. The sense of belonging or security that they create consists in being cocooned in a web of messages rather than finding a soulmate. That way, they hope, the vexing problem of how to achieve a livable balance between freedom and security will disappear.

Fifty years ago, Vernon points out, people relied on their local neighbourhood. Now our ties are looser and we can roam colleges, jobs, cyberspace, lovers, rather than being tied to what have been called communities of fate. Now we live in communities of choice, where we can not only select our closest spiritual partners, but negotiate how close or how distant our relationships should be. We live in looser, more creative and as a result happier times. Or so goes the argument.

The Los Angeles Times recently published a eulogy to the networking skills of young Californians who become friends on such internet sites as MySpace, Friendster and Facebook. Part of the appeal is the low-risk way children meet and develop networking skills. "Kids may not be climbing career ladders, but they are already adept at making social contacts, sharing them, manipulating and using them," wrote Carol Mithers, contending that children's online networking skills "just appear one day, instinctively, and fully formed", which comes as a shock to parents who aren't computer-savvy. "There was a time," Mithers added, "when advice on getting ahead introduced the novel notion of 'networking', pursuing success by building on and taking advantage of one's personal connections. Today the thought that anyone might need such instruction seems so 20th-century."

Vernon is sceptical. "The tone is one of belief, or perhaps hope, that friendship will come into its own: it is elastic enough to connect us across the web of complex lives and strong enough not to snap. But is it? It seems to me that while many people, at both a personal and a social level are turning to friendship, few are asking just what it is."

There's a difference, for a start, between networking and friendship. Or at least there used to be - the distinction is frequently elided in books aimed at helping people to make the best of their social capital. The former producer of the BBC's Any Questions? Carole Stone claims to have 12,000 friends in her address book. Her self-help manual Networking: the Art of Making Friends is billed as a practical guide to help you multiply, mix and match friends. But are these people really friends? Or merely useful acquaintances? Perhaps that is symptomatic of how some people have decided what friends are. Friends are not intrinsically valuable but for something - be it advancement, picking up the kids from school, or to silence the mounting existential scream of solitude.

Ralph Waldo Emerson thought otherwise: friends were desirable in themselves. "When a man becomes dear to me I have touched the goal of fortune," he wrote. Of friendships themselves, he said: "When they are real, they are not glass threads or frostwork, but the solidest things we know." How wonderful! How desirable! But we temperamental Taoists don't value that solidity. Fear of risk exposure to a relationship that may not pay dividends in terms of pleasure or security means that we don't commit. It is an enormous struggle to accept that there are fundamental things like families, love and sex, that don't obey economic rules.

In the Nicomachean Ethics, Aristotle divided friendship into three types. The first is based on brief pleasure, favoured by the young; the second on utility, typically involving commercial transaction and often temporary; the third and perfect friendship is based on goodness. It is rare and needs time and intimacy to flourish. Once it does, though, it is permanent. "It is those who desire the good of their friends for the friends' sake that are most truly friends, because each loves the other for what he is and not for any incidental quality," wrote Aristotle.

This sort of friendship is morally excellent: it allows us to develop a shared idea of the good and altruistic emotions of sympathy, concern and care. And it has another value. "To stand in true relations with men in a false age is worth a fit of insanity, is it not?" wrote Emerson. Instead of treating people as means to advancement or pleasure, to treat them as ends might liberate us from our benighted networking zeitgeist.

Chimpanzees have not achieved Aristotle's exalted friendship of goodness, if research published last month by the University of California is to be believed. The study of 29 chimps found that they failed to help others in their social group even when to do so would cause no inconvenience. They were presented with two reward options: the first allowed a chimp simply to serve itself with food; the second secured the chimp the same reward but also delivered food to another chimp next door. They were no more likely to choose the latter option, despite having been living together for 15 years. Even human SDFs are not this parlously asocial.

Quite possibly these chimps were so depressed at being kept in captivity that, unlike Carole Stone, they were unable to develop the sustaining bonds that even primates need to get on in society. And yet primates, for tens of millions of years, have done just that. If what we call friendship today is an evolutionary mutation of what our ancestors did 30m years ago, then we ought to consider what precisely they got up to. This is what the psychologist Robin Dunbar does in his book Grooming, Gossip and the Evolution of Language. Originally, social bonds were established by primates spending up to a quarter of their time grooming each other. Apes needed such social support so they didn't become something else's dinner.

The groomer offered investment of time and gave the groomee pleasure. It was a much more reliable bond than the one Stone proposes; the free rider problem, whereby someone exploits social cooperation for their own benefit and gives nothing back, was unknown in ancient primate society.

We have changed in the past 30m years: such physical intimacy is rare among friends (when did you last pick ticks for six hours?), but we depend on friendship more than ever for social sustenance. Somewhere between Stone's 12,000 friends and the friendless Californian chimps are the rest of us. The average Briton, according to a recent survey by Professor Ray Pahl, has 18 friends.

Pahl, a sociologist at Essex University's Institute for Social and Economic Research, wrote that men and women have suffered in our modern age. "In what is perceived to be a more unstable and fluctuating world, men were less likely to expect to find close friends at work; the occupational communities had gone and increasing competition meant that colleagues at work became potential rivals. For women, survey evidence demonstrated that their regular contact with family and friends declined from the mid 1980s. This was attributed partly to the pressures consequent upon the successful juggling of family and work responsibilities." Some sociologists suggest that friendship is no longer possible at work.

When Pahl was written up in the Sunday Times recently under the headline We Get by With Help From 18 Friends, his thesis was mistakenly portrayed as suggesting that each of us needs that number of intimates to draw sufficient emotional and practical support. And in website after website this notion that you need 18 friends for optimum lifestyle has gained credence.

Pahl, whose research will appear shortly in a book called Rethinking Friendship: Hidden Solidarities Today, is trying to map the complicated terrain of modern friendship. He suggests seven types of friendship, four more than Aristotle. But then modern Britain is a more complex place than ancient Greece. They are:

1 A friend-like community, where a person depends more on friends than family, with close friends at the centre of the network, and more casual ones and relatives further way.

2 A friend-enveloped community, with close relatives (spouse and offspring) at the centre, and a larger group of friends around the family.

3 A family-like community in which family members outnumber friends.

4 Family-dependent - family outnumber friends.

5 Partner-focused, in which a couple keep friends and relatives at a distance.

6 Neighbourhood-focused, often formed by older people in close neighbourhood communities.

7 Professional-dependent, again often formed by older people whose most important friends are carers or social workers.

Across these types the average number of friends is 18. The survey by Pahl and his co-author, Liz Spencer, indicates family structures and neighbourhood ties may not be eroding quite so fast as some have suggested. Communities of fate have not yet been replaced by communities of choice. It also indicates that we might be members of different friendship communities at different times of our lives.

The survey suggests friendly solidarity has not been completely destroyed by selfish consumerism. Friendship has mutated rather than died, and become more instrumental than our utopian thinkers imagined it to be at best. Whether it can help us feel less insecure in these dislocated times is a moot point. But at least we are not yet as friendless as the captive chimpanzees of California.

· Mark Vernon's The Philosophy of Friendship is published by Palgrave Macmillan. Ray Pahl's On Friendship is published by Polity. Pahl and Liz Spencer's study Rethinking Friendship is forthcoming